This invention relates generally to analog receiver circuits and more particularly to a radio frequency variable gain mixer.
An example of a classic active mixer is shown in FIG. 1. The mixer 10 is comprised of a mixer core 13, which includes transistors Q1-Q4, and an RF input section 15. The mixer 10 is commonly known as the "Gilbert Mixer" after the inventor of the present application. In fact, a prior art search at the time of the invention thereof produced a reference, U.S. Pat. No. 3,241,078 issued to Jones, which showed essentially the same circuit, although it was described as a "synchronous detector," and did not seem to envision its utility in mixer applications.
The operation of the mixer 10 is as follows. In the absence of any voltage difference between the base of Q5 and Q6, the collector currents of these two transistors are essentially equal. Thus, a voltage applied to the LO port 12 results in no change of output current. Should a small DC offset voltage be present at the RF port (e.g., due to a mismatch in the emitter areas of Q5 and Q6), this will only result in a small feed through of the LO signal V.sub.LO to the IF output port, which will be blocked by a first IF filter (not shown). Conversely, if an RF signal V.sub.RF is applied to the RF port 14, but no voltage difference is applied to the LO port 12, the output currents will again be balanced. A small offset voltage (due now to emitter mismatch in Q1-Q4) may cause some RF signal feed through to the IF port; as before, this will be rejected by the IF filter. Thus it is only then when a signal supplied to both the LO port 12 and the RF port 14 that a signal appears at the IF port 16.
FIG. 2 is a graph showing the relationship between the RF input voltage (V.sub.RF) and output voltage (V.sub.IF) for the mixer 10, where the LO signal V.sub.LO is held at a DC value. As is known in the art, the relationship between V.sub.RF and V.sub.IF can be described by the hyperbolic tangent (tanh) function. As can be seen in FIG. 2, there is essentially a linear relationship between the input and output signals for a certain small operating range, generally between points 22 and 23. This is due to the linear transconductance (g.sub.m) of the RF input section over that operating range. The g.sub.m of transistors Q5 and Q6, however, becomes increasingly nonlinear outside this small operating range causing the slope of the response to decline past points 22 and 23. The nonlinear operating characteristics of g.sub.m effectively compresses the input signal, inducing unwanted inter-modulation products in the mixer output signal. Emitter degeneration resistors in the emitters of Q5 and Q6 can be used to improve the linearity but this degrades the noise performance of the mixer.
Another problem with the mixer shown in FIG. 1 is the switching noise generated by the core transistors (e.g., Q1-Q4) as they switch between their "on" and "off" states. This switching noise compromises the spectral integrity of the mixer output. Referring to FIG. 3, an upper graph 26 shows a local oscillator (V.sub.LO) signal fed into the differential pair shown in FIG. 2. A lower graph 27 shows noise bursts 28 created by the amplifier during transition periods when the V.sub.LO signal changes between high and low states. To reduce noise bursts 28, the mixer core transistors need to be turned on more quickly to supply more charge to their base terminals. However, more current is then required to remove the charge from the base of Q1, which makes this approach difficult to implement for high frequency applications. Accordingly, needs remain for a low noise mixer circuit with improved linearity and for providing the base charge more rapidly to and from the LO port.